In recent years, social demand with respect to energy conservation and concerns about environmental problems have increased, and emission of carbon dioxide is now controlled more and more rigorously throughout the world. In the midst of such a trend, demand has increased for reducing fuel consumption of automobiles. In order to satisfy such demands, performance of tires must be enhanced; particularly, rolling resistance must be reduced. Previously, optimization of the structure of tires was investigated for reducing rolling resistance. However, at present, rolling resistance is generally reduced through employment of a low-heat-buildup rubber composition.
In order to produce such a low-heat-buildup rubber composition, a variety of modified rubbers containing silica or carbon black serving as a filler have been developed. Among the techniques for producing such modified rubbers, Japanese Patent Publication (kokoku) Nos. 6-53763 and 6-57767 and other documents disclose particularly effective approaches in which the polymerization active end of a conjugated diene based polymer produced through anionic polymerization in the presence of an organic lithium is modified with an alkoxysilane derivative having a functional group interacting with a filler.
Although, these approaches are generally effective for polymers having a stable polymerization living end, modification of a rubber composition containing silica or carbon black has not been satisfactorily attained. In addition, when a conventional modification technique is employed, in many cases, branching of the backbone of the polymer is insufficient. Therefore, when such a modified rubber is used in practice, problematic cold flow occurs. In this case, partial coupling is performed so as to prevent cold flow and, as a result, the effect of modification is reduced.
Thus, an approach for overcoming the above drawbacks and enhancing the effect of modification has been proposed. WO 03/087171 discloses a method in which a condensation-accelerating agent is added to a reaction system during modification of the active end of a conjugated diene based polymer with an alkoxysilane. Although the method successfully prevents loss of a silica filler in the rubber composition, loss of a carbon black filler cannot be satisfactorily prevented.    Patent document 1: Japanese Patent Application Examined Publication No. 53763/1994    Patent document 2: Japanese Patent Application Examined Publication No. 57767/1994    Patent document 3: WO03/087171 pamphlet